To elaborate, adding two negatives together doesn't make a postive. Multiplying two negatives together makes a positive. In other words, if you negate a negative number, you end up with a positive one (because the way to negate something is to multiply it by -1). This is the analogy the the book was trying (but failed) to make. In English it is similar: if you negate a statement that is already phrased as a negative, then the two cancel each other out. E.g. "I ain't got no money," which in correct English would be, "I haven't got no money." But if you haven't got no money, then you must have some money. So you can see that the meaning of the sentence with the double negative is the same as the meaning of the sentence, "I have got some money." So, double negatives are deemed incorrect in English (i.e. shouldn't be used) because there are only two possible intended uses:

1. An incorrect way of phrasing something as a negative

2. A confusing and unnecessarily convoluted way of phrasing something as an affirmative.

To elaborate, adding two negatives together doesn't make a postive. Multiplying two negatives together makes a positive. In other words, if you negate a negative number, you end up with a positive one (because the way to negate something is to multiply it by -1). This is the analogy the the book was trying (but failed) to make. In English it is similar: if you negate a statement that is already phrased as a negative, then the two cancel each other out. E.g. "I ain't got no money," which in correct English would be, "I haven't got no money." But if you haven't got no money, then you must have some money. So you can see that the meaning of the sentence with the double negative is the same as the meaning of the sentence, "I have got some money." So, double negatives are deemed incorrect in English (i.e. shouldn't be used) because there are only two possible intended uses:

1. An incorrect way of phrasing something as a negative

2. A confusing and unnecessarily convoluted way of phrasing something as an affirmative.

Cepeid gave good explanation but to add,
For those learning to be puritans of English:
E.g. "I ain't got no money,"
"ain't got " - ain't is not an (acceptable) word in proper English.

And the "got" is poor colloquial spoken English, and should be eliminated.
"I have got some money " --> "I have some money".

Intended use number 3.
Double negatives are used in English to also provide emphasis to certain thoughts or ideas, that without, lacks what a speaker or writer wishes to convey. At times the double negatives do not cancel out as one wishes to think.

Staff: Mentor

Intended use number 3.
Double negatives are used in English to also provide emphasis to certain thoughts or ideas, that without, lacks what a speaker or writer wishes to convey. At times the double negatives do not cancel out as one wishes to think.